The photographic industry is working with photographic materials which have to respond to light with a well-defined minimum energy content. This light energy should trigger a chemical or physical activity leading to a change in the illuminated material which is immediately visible or only intrinsically present and can be visualized afterwards by an additional treatment also called processing step. However already for a long time there exists a strong and an even increasing demand for photosensitive materials that have improved sensitivity, that is for materials that respond to a decreasing amount of light energy. One of the most interesting possibilities is found in photosensitive materials wherein the primary light-activated change exists on an atomic or molecular base which in a secondary step can be multiplicated by several orders of magnitude in order to visualize the first light interaction in the material. This `two-step` image formation mechanism is for instance encountered in silver halide materials which form the main subject of the present invention. It will be clear that sensitivity in this type of materials is determined by the efficiency in which the different steps between the light interaction with the silver halide and the formation of the visual image can proceed. This invention will be specially focused on the way in which the first molecular light-induced change in silver halide crystals (also called latent image) is realized. This change after development gives rise to the formation of a visual image.
The efficiency of the latent image formation depends on many factors and can therefore be influenced in many different ways. The best result is realized if each photoelectron in the silver halide crystal reaches the deepest electron trap while forming the latent image. This means that the recombination between holes and electrons that are created after light absorption is prevented as much as possible. Many solutions are proposed but all of them have a limited result. One can primarily try to lower the depth of electron trap in order to increase the capture probability. Chemical sensitization with for instance sulphur, gold, selenium and other compounds or combinations thereof is mostly used for this purpose.
Another way in preventing the recombination of holes and electrons after formation is the temporary interception of these species at local traps with intermediate trap depth. This can be done by creating a distortion internally in the crystal lattice for instance by the local incorporation of an increased amount of iodide in the core or in a small zone of the grain. Although this method leads to a sensitivity gain by decreasing the electron-hole recombination, another important feature like developability, which is desired in modern photographic materials is deteriorated by the presence of the iodide.
Increasing the sensitivity of a silver halide emulsion can also be realized by increasing the efficiency of electron transfer from the spectral sensitizer to the silver halide grain which principally can be carried out with a supersensitizer.
Looking at the activity of the sensitizing dye, sensitivity gain can also be realized by decreasing the dye desensitization which is evolved by increasing dye concentration at the grain surface. This can for instance be done by combining a electron donating compound like ascorbic acid with specific cyanine and merocyanine dyes as described in U.S. Pat. No. 4,897,343. Electron-donating compounds that are attached to a sensitizing dye or a silver halide absorptive group have also been used to get a additional sensitizing effect. Examples are described in U.S. Pat. Nos. 5,436,121, 5,478,719 and 4,607,006.
Another interesting alternative in order to decrease recombination is introducing hole traps like silver clusters or some metal ions or complexes in the silver halide crystal. One of the metal salts which can be used comprises iron as is described, for instance, in U.S. Pat. Nos. 5,166,044 and 5,420,001, where the iron ions are incorporated in silver halide emulsions rich in silver chloride (&gt;90 mole % of AgCl). The sensitivity increase is caused by a ferro or ferri salt, wherein the (inorganic or organic) anion can be freely choosen. The choice of the anion does not influence the desired effect as has extensively been demonstrated. Further hole-trapping entities which were already mentioned are silver clusters. These can be created in the crystals by reduction sensitization which is realized by treating the emulsion during the precipitation with a reductor like tin compounds, polyamine derivatives, hydrazines, ascorbic acid and analogues, etc. or by well defined pH- and/or pAg-conditions without using reducing substances. So in U.S. Pat. No. 3,892,574 a method has been described, wherein during precipitation of the silver halide or before or during physical ripening small silver specks (which do not give spontaneously developable fog) are created in reducing conditions. The same can be said for silver halide preparation methods as described in U.S. Pat. No. 3,957,490, where at the end of a reduction period an oxidant is introduced in the silver halide emulsion before chemical sensitization. Most of the methods concerning silver cluster formation by reduction sensitization give (more or less) rise to fog which is known as a serious problem. Experimental evidence has been found that these silver clusters can easily be formed on {111}-AgBr crystal faces while it becomes difficult for instance on {100}-AgBr or on {100}-AgCl crystal faces. For this reason a more general method is needed which can form hole-trapping entities in all kind of silver halide crystals in order to improve the sensitometric characteristics.